Distinct differences in physiological and biochemical responses of juvenile Saccharina latissima sporophytes (sugar kelp, Phaeophyceae) to acute and chronic warming

Rising ocean temperature, driven by climate change, poses a significant threat to kelp forests by subjecting them to both acute warming, i.e. marine heatwaves, and chronic warming, i.e. gradual increase in sea surface temperature. This study investigated physiological and biochemical responses of juvenile Saccharina latissima sporophytes to acute (1-day to 10-day) and chronic (20-day to 40-day) warming from 11 °C to 21 °C, followed by exposure to 25 °C. Physiological responses comprised survival, relative growth rate and photosynthetic efficiency. Biochemical response analyzed were total organic carbon, total nitrogen content as well as the carbon-to‑nitrogen (C:N) ratio. Attenuated Total Reflectance Fourier-Transform Infrared spectroscopy was used to investigate possible temperature-driven shifts in biochemical composition. Acute warming impaired physiological performance and reduced survival of juvenile S. latissima sporophytes. Chronic warming led to elevated carbon and nitrogen reserves, increased fucoidan and protein levels, and enhanced photosynthetic performance. Improved heat tolerance of juvenile S. latissima sporophytes was observed only in sporophytes previously exposed to 25 °C only after prior chronic warming treatments. These results demonstrate that while exposure to chronic (gradual) temperature increases may allow S. latissima to acclimate, events exceed their physiological limits, leading to low survival. It seems that especially acute warming ultimately determines the presence or distribution of S. lattisima . • Acute warming (mimicking marine heatwaves) is an ultimate threat to Sacchrina latissima forest’s existence. • Acute warming impaires physiological performance and reduces survival of juvenile S. latissima sporophytes . • Chronic warming (mimicking gradually increasing sea surface temperatures) leads to elevated carbon and nitrogen reserves, increased fucoidan and protein levels, enhanced photosynthetic performance and improved heat tolerance of juvenile S. latissima sporophytes . • Moderate thermal stress to S. latissima can accumulate over time, leading to sudden die-offs after exposure to more extreme temperatures.